1. Field of the Invention
The present invention relates to a magnetoresistive sensor in which the magnetization of a pinned magnetic layer is firmly fixed by the uniaxial anisotropy of the pinned magnetic layer.
2. Description of the Related Art
In conventional magnetoresistive sensors, the magnetization of a pinned magnetic layer is generally fixed by utilizing an exchange coupling magnetic field generated between the pinned magnetic layer and an antiferromagnetic layer, for example, having a thickness of about 20 nm.
FIG. 12 is a schematic view of the structure of a magnetoresistive sensor of a conventional CPP type. Specifically, above and below a free magnetic layer 1 are disposed upper and lower pinned magnetic layers 3, 3 and upper and lower antiferromagnetic layers 4, 4 with upper and lower nonmagnetic material layers 2, 2 interposed between the free magnetic layer 1 and the upper and the lower pinned magnetic layer 3, 3. This multilayer film is sandwiched between a lower electrode 5 and an upper electrode 6.
The term “current perpendicular to the plane (CPP) type” used herein means a structure in which an electric current flows in the direction perpendicular to the transverse surfaces of each layer in the multilayer film. On the other hand, the term “current in the plane (CIP) type” used herein means a structure in which an electric current flows in the direction parallel to the transverse surfaces of each layer in the multilayer film.
Although most of currently-used magnetoresistive sensors are of the CIP type, magnetoresistive sensors of the CPP type can be advantageously downsized and have an improved playback output, as compared with the magnetoresistive sensors of the CIP type. Thus, the magnetoresistive sensors of the CPP type are expected to comply with future higher recording densities.
However, in the structure shown in FIG. 12, the antiferromagnetic layers 4, 4 have a specific resistance as high as about 200 μQ•cm2 (or more). Thus, when an electric current flows between the lower electrode 5 and the upper electrode 6, the antiferromagnetic layers 4, 4 act as heat sources, generating a Joule heat. The generated Joule heat enhances phonon scattering due to lattice vibration or the electromigration of conduction electrons in the adjacent pinned magnetic layers 3, 3, the nonmagnetic material layers 2, 2, and the free magnetic layer 1.
As a result, it was found that, in the magnetoresistive sensors of the CPP type, the GMR effect represented by the resistance change per unit area (ΔR·A) could not be improved.
Furthermore, in the magnetoresistive sensors of the CIP type, the antiferromagnetic layers 4, 4 principally cause shunt loss of a sense current. Providing for higher recording densities of recording media, the magnetoresistive sensor is suffered from a low output due to the shunt loss of the sense current.
Furthermore, in the magnetoresistive sensors of both the CIP type and the CPP type, a very thick antiferromagnetic layer results in an increased distance between shielding layers on and under the multilayer film, making the structure unsuitable for the recording media having higher linear recording densities.
In a magnetoresistive sensor disclosed in Japanese Unexamined Patent Application Publication No. 8-7235 (hereinafter referred to as JP-08007235-A), the antiferromagnetic layer is removed from a multilayer film, and the magnetization of a pinned magnetic layer is fixed by the uniaxial anisotropy of the pinned magnetic layer.
In the JP-08007235-A, a pinned ferromagnetic layer 70 (pinned magnetic layer) is stacked on a base of a buffer layer 62 made of tantalum. However, a principle by which the magnetization of the pinned ferromagnetic layer 70 can be firmly fixed using the buffer layer 62 made of tantalum is not clear. Since tantalum tends to be amorphous, the buffer layer 62 made of tantalum, in practice, will not firmly fix the magnetization of the pinned ferromagnetic layer 70 (pinned magnetic layer). Furthermore, since tantalum has a high specific resistance, when the structure according to the JP-08007235-A is applied to the magnetoresistive sensors of the CPP type, the buffer layer 62 will act as a heat source like the conventional antiferromagnetic layer. This causes scattering independent of the spin of a conduction electron and therefore the GMR effect will not be improved. Thus, the structure according to the JP-08007235-A cannot be employed as such.
Furthermore, in the JP-08007235-A, a ferromagnetic film 74 away from the buffer layer 62 is not “self-pinned”. In the structure according to the JP-08007235-A, a ferromagnetic film 72 and the ferromagnetic film 74 are liable to vary with an external magnetic field while maintaining antiparallel magnetization.
In the JP-08007235-A, it should be avoided to change the conventional structure of an interface between the ferromagnetic film 74 and a copper spacer layer 63 to enhance the fixed magnetization of the ferromagnetic film 74, because this change deteriorates a CIP-GMR effect.
As disclosed in the JP-08007235-A, a layer 73 between the ferromagnetic films 72 and 74 is made of ruthenium (Ru). As described in paragraph [0031] in the JP-08007235-A, the magnetizations of the ferromagnetic films 72 and 74 are aligned antiparallel with each other owing to an antiferromagnetic exchange coupling through the Ru antiferromagnetically coupling film 73.
The Ru antiferromagnetically coupling film 73 has a function of increasing the magnetostrictions of the ferromagnetic films 72 and 74 to some extent. The increased magnetostrictions help to enhance the uniaxial anisotropy and the fixed magnetization of the pinned magnetic layer, as described below. However, since the magnetostriction-enhancing effect of the Ru antiferromagnetically coupling film 73 is not so large, the magnetizations of the ferromagnetic films 72 and 74 made of Co cannot be firmly fixed in any event in the laminated structure of Ta (buffer layer 62)/Co (ferromagnetic film 72)/Ru (antiferromagnetically coupling film 73)/Co (ferromagnetic film 74) as in the JP-08007235-A1.